Magnetic memories, particularly magnetic random-access memories (MRAMs), have drawn increasing interest due to their potential for high read/write speed, excellent endurance, non-volatility and low power consumption during operation. An MRAM can store information utilizing magnetic materials as an information recording medium. Some magnetic memories write to the magnetic material using a current. One such magnetic memory programs magnetic junctions using spin-orbit interaction (SO) torque.
SO torque-based memories, such as a SO torque magnetic random-access memory (SOT-MRAM), utilize conventional magnetic tunneling junctions (MTJs) in conjunction with a line having a high spin-orbit interaction (hereinafter SO line). The conventional MTJ includes a pinned (or reference) layer, a free layer and a tunneling barrier layer between the pinned and free layers. The MTJ typically resides on a substrate and may include seed and capping layer(s) as well as an antiferromagnetic (AFM) layer. The reference layer and the free layer are magnetic. The magnetization of the reference layer is fixed, or pinned, in a particular direction. The free layer has a changeable magnetization. The reference layer and free layer may have their magnetizations oriented perpendicular to the plane of the layers (perpendicular-to-plane) or in the plane of the layers (in-plane). The SO line is adjacent to the free layer of the conventional MTJ. The high spin-orbit interaction may be due to a bulk effect of the material itself (spin Hall effect), due to interfacial interactions (Rashba effect), some other effect and/or some combination thereof.
In conventional SO memories, writing is performed by driving a current in-plane (CIP) through the SO line. If the free layer magnetic moment is stable in-plane, then the in-plane SO torque alone can switch the free layer between stable states. Thus, a current driven through the adjacent SO line generates an SO torque that may switch the direction of magnetization of the free layer without additional switching mechanism(s). In contrast, if the free layer has a magnetic moment that is stable perpendicular-to-plane, then an additional torque is used. Since the spin orbit torque is in-plane, in order to reliably switch the magnetic moment using the in-plane current, a symmetry breaking additional torque is required, and can be achieved by either modest external magnetic field, an in-stack magnetic bias, or STT torque through MgO barrier. The in-plane current develops an SO torque, which can be used to rotate the free layer magnetic moment from vertical to near in-plane direction. Switching to the desired direction is completed using the external magnetic bias or STT current. For example, the external magnetic field, an additional AFM layer or biasing structure may magnetically bias the free layer to complete switching to the desired state.
Although the conventional magnetic junction may be written using spin transfer and used in a spin transfer torque random access memory (STT-RAM), there are drawbacks. In general, SO torque is not an efficient mechanism for switching the free layer. Stated differently, the SO angle (measure of this efficiency of SO torque) is generally small. Thus, a high write current may be required for writing. In addition, the spin current in regions not adjacent to the magnetic is not used in writing. Thus, this spin current may be wasted. Memory cells using SO torque may have a large footprint because a three-terminal device may be used for write and read operations. Perpendicular magnetic moments in the layers of the magnetic junction may also not be usable in some embodiments. Thus, scalability may be limited. Consequently, a mechanism for improving SO torque magnetic devices is still desired.